The present invention relates to a new and improved method of automatically controlling a continuously operating pressure filter.
In its more particular aspects the method for the automatic control of a pressure filter according to the invention is of the type including the steps of introducing a flow of a suspension at a first rate of flow into a filtering chamber formed in the pressure filter and comprising filtering walls; removing a flow of a filtrate at a second rate of flow from the filtering chamber; removing a flow of a product at a third rate of flow from the filtering chamber; maintaining a constant tolerated operating pressure within the filtering chamber, wherein the transmission or throughput capacity of the filtering walls in the filtering chamber gradually decreases with increasing operating time of the pressure filter.
Filters of this type are known, for example, from U.S. Pat. No. 3,797,662, German Patent Publication No. 2,054,968, U.S. Pat. No. 4,066,554, and German Patent No. 2,558,682.
Such pressure filters comprise an annular filtering chamber defined by cylinder-shaped filtering walls. The filtering walls may be rotated relative to each other, resulting in the formation of turbulence in the suspension to be separated and contained in the filtering chamber, so that a certain self-cleaning effect is obtained at the filtering walls. Thus, a solid cake is more slowly formed on the filtering surfaces in comparison to other pressure filters. But even in pressure filters of this kind the transmission or throughput capacity of the filtering walls continuously decreases with progressive operating time and with the growing solid cake layer. During the separation of a suspension into a filtrate and a product, which is a concentrated suspension, a constant quality of the product is required during the operating time. In other words, a constant ratio of the proportion of dry solids to the liquid contained in the product is desired.
Up to now this postulate has remained unfulfilled when using conventional regulating methods. A constant amount of suspension per unit time, that is a constant flow of suspension having constant quality, i.e. having a constant ratio of the amount of dry solids and the amount of liquid therein is introduced into the pressure filter. In the filtering or filtration chamber a constant operating pressure is maintained by restricting the outflow of the product. With continuously decreasing transmission capacity of the filtering walls a continuously decreasing amount of the filtrate to be separated is removed through the filtering walls, so that a continuously greater amount of liquid remains in the filtering chamber and is removed as a proportion of the product together therewith. This implies that the product becomes continuously diluted with a continuously deteriorating ratio of the proportion of dry solids and liquid in favor of the latter. To a certain extent the continuously deteriorating quality of the product is tolerated in the methods presently in use. Upon reaching the limit of tolerance the operation is stopped and the transmission or throughput capacity of the filtering walls is restored, for example, by means of flushing and the operation is again resumed.